Underwater theme

Stages with an underwater theme have significant sections set underwater. Enemies tend to be modeled on fish or other aquatic creatures, but the main obstacle in underwater levels is the water itself. Since Sonic cannot swim or float, he runs along the bottom of the levels just like on land, except that he moves slower and must regularly replenish his air supply or else die by drowning. Air can be obtained by surfacing or by swallowing air bubbles. Partly for this reason, underwater levels are often the most difficult levels in a Sonic game.

Other common elements in underwater levels include water spouts that rocket Sonic to new locations, fast-moving flows of water that Sonic must either ride or resist, and rising water levels that Sonic must keep ahead of as a sort of time limit to certain sections.